Reproducible sputtering and properties of Y-Ba-Cu-O films of various thicknesses

One goal of this work was to develop a reproducible method of preparing high quality Y-Ba-Cu-O superconducting films and to study their properties versus thickness. This was accomplished by rf diode sputtering from a single target. Twenty-seven depositions were made using a target containing 8.9-at. % Y, 37.3-at. % Ba, and 53.8-at. % Cu. Film thicknesses ranged from 0.09 to 2.4 μm. The film compositions obtained were 15.6±1.0-at. % Y, 35.8±1.0-at. % Ba, and 48.7±1.7-at. % Cu for the mean and standard deviation. The films were amorphous as-deposited and crystallized by annealing in O2 at 915 °C. X-ray diffraction, transmission electron microscopy, and scanning electron microscopy indicated that, on (100)SrTiO3 substrates, films with thickness less than ∼0.25 μm were epitaxially oriented with their c axis perpendicular to the substrate. Films on (110)SrTiO3 were oriented with their c axis parallel to the substrate. On (100)SrTiO3, zero resistance was achieved in 30 samples from 27 runs at 85.6±1.4 K with a transition width (10%–90%) of 1.8±1.1 K independent of thickness. Results for deposition on a variety of other substrates were more variable. Diamagnetic shielding of up to 38% was calculated from the initial slope of M vs H. This low value was attributed to the presence of second phases and a significant diffusion layer thickness, both observed by transmission electron microscopy. Critical currents, measured by transport using a four-point probe, reached 8.1×105 A/cm2 at 77.35 K for a 0.2-μm film deposited on (100)SrTiO3 . Comparable values were obtained by calculation from the M-H hysteresis loop, from which we infer that there are either continuous epitaxial sheets of c axis oriented 123 or, if there are grain boundaries, the coupling across them is strong.